Start with a partial water change, not a full drain

The fastest way to ruin a small aquarium is to empty it completely and scrub it clean. That wipes out the bacteria that keep ammonia and nitrite from poisoning your fish. Instead, remove 25 to 30 percent of the water, clean what you can reach without disturbing the substrate, and refill with fresh water at the same temperature.

Small aquariums — typically 10 gallons or fewer — need cleaning more often than larger tanks because waste builds up faster in less water. Plan on a partial water change every week, and a deeper clean of the filter and decorations every two to three weeks. The smaller the tank, the more frequent the maintenance.

Key Takeaways

  • Remove 25 to 30 percent of the water each week; never drain and refill completely, as this kills the bacteria that protect your fish.
  • Clean the filter media in old tank water, not tap water, to preserve the beneficial bacteria living in it.
  • Scrape algae off the glass with a plastic scraper or old credit card, and wipe decorations with a soft brush or cloth.
  • Match the temperature of fresh water to the tank before adding it, and add it slowly to avoid shocking your fish.
  • Small tanks need weekly partial changes and deeper cleaning every two to three weeks because waste accumulates faster.

Remove water and clean the glass in one step

Use a siphon or small aquarium gravel vacuum to remove the water. A siphon is a tube you fill by hand and place so one end sits in the tank and the other drains into a bucket; gravity does the work. A gravel vacuum is a wider tube with a bulb at the top — you squeeze the bulb to start the flow, and it sucks debris from the substrate as it drains.

While the water level is lower, scrape algae off the glass with a plastic algae scraper, an old credit card, or a soft cloth. Do not use anything abrasive or metal, which scratches acrylic or glass. Wipe the inside surfaces where algae builds up, usually the front and sides. Leave the back and corners alone unless they are visibly caked — the algae there feeds microorganisms your fish depend on.

Clean the filter without killing the bacteria

The filter cartridge or media is where most of your beneficial bacteria live. If you rinse it under tap water, the chlorine kills them and you lose your biological buffer. Instead, squeeze or swish the filter media in a bucket of the water you just removed from the tank. This cleans out trapped waste while keeping the bacteria alive.

If your filter has a foam insert, rinse it gently in old tank water until the water runs clear. If it has a cartridge, check whether it is disposable or reusable — the packaging or your filter manual will say. Disposable cartridges go in the trash after a few weeks; reusable ones last months and just need rinsing. Replace the media while it is still damp, and reassemble the filter before refilling the tank.

Refill with fresh water at the right temperature

Cold water shocks fish and can kill them. Before you add anything back to the tank, measure the temperature of the water already in it with a thermometer. Fill a bucket with fresh dechlorinated water — let tap water sit for 24 hours, or use a dechlorinator product — and warm or cool it to match the tank temperature within a degree or two.

Pour the water in slowly, aiming at a decoration or the side of the tank rather than directly at the fish. This gives them time to adjust and prevents the current from scattering gravel or uprooting plants. If you are adding water to a very small tank (5 gallons or less), pour even more slowly — the temperature can swing fast in shallow water.

Clean decorations and plants on a rotating schedule

Decorations collect algae and waste, but you do not need to clean them every week. Every two to three weeks, remove one or two items and scrub them with a soft brush or cloth under running water. Rocks, plastic plants, and driftwood all respond to gentle scrubbing. Avoid soap or chemicals — plain water is enough.

Live plants do not need scrubbing; just remove dead leaves by hand. If algae is coating a live plant thickly, you can gently rub it off with your fingers or a soft cloth, but leave some algae behind — it is food for some fish and invertebrates. Rotate which decorations you clean so you are not disrupting the tank too much in any single week.

Test water quality if the tank looks cloudy or smells bad

A properly maintained small aquarium should be clear and odorless. If the water turns cloudy or develops a smell, something is wrong. Cloudiness usually means a bacterial bloom — too much waste is feeding bacteria faster than your filter can handle. A bad smell means ammonia or hydrogen sulfide is building up, which is toxic to fish.

If this happens, do a larger water change — 40 to 50 percent instead of 25 — and clean the filter more thoroughly. Check that the filter is running and that the intake is not clogged. If you have a test kit, check ammonia and nitrite levels; both should be zero. If they are not, increase water changes to twice a week until the levels drop. If the problem persists after a week of frequent changes, the tank may be overstocked or the filter may be too small for the number of fish.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Tap water contains chlorine, which kills fish and the bacteria in your filter. Let it sit in an open bucket for 24 hours before adding it to the tank, or use a dechlorinator product that removes chlorine when ready. Some areas also have chloramine, which requires a different dechlorinator — check your local water report if plain dechlorinator does not work.

How often should I replace the filter cartridge?

Disposable cartridges typically last three to four weeks before they clog and restrict water flow. Reusable foam or sponge filters last much longer — months — and just need rinsing in old tank water. Check your filter manual or the cartridge packaging to see which type you have. When flow slows noticeably, it is time to replace or clean.

What if I see white or brown film on the water surface?

That is usually a bacterial or fungal film caused by excess organic matter. Skim it off with a small net or paper towel, then do a 30 percent water change and clean the filter. Make sure the filter intake is not blocked and that you are not overfeeding — uneaten food decays and feeds the film. If it returns within days, increase water changes to twice weekly.

Is it safe to clean the tank while the fish are in it?

Yes. Removing the fish stresses them more than a gentle cleaning does. Work slowly, avoid sudden movements, and do not scrub decorations right next to where they are hiding. The only time to remove fish is if you need to catch a sick one or if you are doing a major tank overhaul — which should not happen in a well-maintained small aquarium.